1. Technical Field
This invention relates generally to pistons for heavy-duty engine applications.
2. Related Prior Art
Pistons for heavy-duty applications include an upper head formed with a ring belt having pin bosses depending therefrom for connecting the piston to a connecting rod of the engine. Such pistons also include a piston skirt that is formed either as one piece with the head and pin boss portions, or formed as a separate structure which is coupled in articulated fashion to the pin bosses via the wrist pin.
In pistons having a unitized skirt, axial loading from the combustion gases is transmitted through the head to the pin boss section. Since the skirt is coupled directly to the head as an extension of the ring belt, the skirt also sees axial loads. Such unitized skirts also tend to be overly rigid, as they are tied directly to the ring belt as one piece. Such inflexible skirts may make sensitive to scuffing or marking of the skirt and cylinder wall during radial thrust loading of the skirt. U.S. Pat. No. 6,155,157 shows such a piston.
Still a further characteristic of forming the skirt as one piece with the ring belt is that the heat from the head is transmitted directly to the skirt, and thus added clearance must be provided between the skirt and cylinder wall to compensate for thermal expansion of the skirt, sacrificing closeness of fit between the piston skirt and cylinder wall.
A piston constructed according to the present invention overcomes or minimizes the above difficulties associated with monobloc pistons.
A piston constructed according to the invention for heavy duty engine applications comprises a piston head having a peripheral ring belt with a lower end and a pair of pin bosses depending from the head portion, each formed with a wrist pin hole. An internal piston cooling gallery is formed in the head and includes a bottom wall coupled to and extending radially inwardly from the ring belt. The piston construction includes a piston skirt coupled directly and immovably to the pin bosses having an upper free end which is spaced in uncoupled relation from the lower end of the ring belt and the bottom wall of the piston cooling gallery.
A method is provided for making such a piston and comprises forming a head portion of the piston having an internal cooling gallery, a peripheral ring belt with a lower end and a pair of depending pin bosses, and unitizing a piston skirt with the pin bosses in direct immovable relation thereto, and having an upper free end of the skirt arranged in spaced uncoupled relation to the lower end of the ring belt.
One advantage offered by such a piston construction is increased flexibility of the skirt, since it is no longer united directly to the ring belt, and thus is free to flex and give under radial loads, thereby decreasing the occurrence of scuffing or marking of the cylinder walls associated with the aforementioned rigid skirts.
Another recognized advantage of uncoupling the skirt from the ring belt is that the intervening space precludes the axial loads imparted to the head by combustion gases from being transmitted to the skirt. Such loads are instead transmitted directly to the pin bosses, isolating the skirt from such axial loads.
Another advantage of the uncoupled skirt configuration is that the intervening space serves as an insulation barrier to prevent direct transmission of heat from the head to the skirt. By isolating the skirt from the heat of the head portion, the skirt is subject to far less thermal expansion and thus can be designed with a closer fit relative to the cylinder wall than the traditional coupled skirts above.
Still another advantage recognized by the subject piston construction is that it requires relatively less material to manufacture and is thus lighter in weight and less costly to produce than traditional monobloc pistons.